There exist a variety of fabrics today which are capable of acting as a barrier to particulate matter, water and other liquids yet which allow water vapor and air to pass therethrough. Such fabrics are commonly referred to as "breathable barriers." Breathable barrier fabrics have been employed in outdoor fabrics, tarpaulins, garments, personal care products, infection control products, as well as numerous other articles. Moreover breathable barrier fabrics are often preferred over non-breathable barrier materials since breathable barrier fabrics allow moisture trapped beneath the fabric to escape as water vapor. Thus, apparel using breathable barriers is generally more comfortable to wear since the migration of water vapor through the fabric helps to reduce and/or eliminate discomfort resulting from excess moisture trapped against the skin.
While a variety of breathable barrier fabrics are known in the art, one particularly useful breathable barrier comprises stretched filled microporous films. Such films are typically filled with particles and then crushed or stretched to form a fine pore network which creates tortuous paths through the film. The film pore network allows gas and water vapor to pass through the film while acting as a barrier to liquids or particulate matter. The amount of filler within the film and the degree of stretching is controlled so as to create a microporous network of tortuous paths which are of a size and/or frequency to impart the desired level of breathability to the fabric. An example of stretched filled film is described in U.S. Pat. No. 4,777,073 issued to Sheth which discloses a stretched filled polyolefin film filled with about 15 to 35% by volume calcium carbonate which can be stretched to about four times its original length.
While filled microporous films are capable of providing good barrier properties and breathability, efficient commercialization and practical applications of such films requires relatively low defect rates. The barrier properties of stretched filled films may be compromised by defects such as macroscopic holes or zones of weakness in the film. This is of enormous concern where the film is intended to act as a barrier to urine, blood or other potentially hazardous materials. However, the process of stretching the filled films to the required degree, while acting to orient the film and also make the film microporous, also has the adverse effect of creating many of the defects. Thus, the process and manner in which such films are produced can have a significant impact upon the number and frequency of defects within the stretched-thinned films. It therefore follows that methods which produce less defects create superior films and have the added benefit of being more efficient in the sense of creating less unusable or defective product.
Unfortunately, the production of stretched filled films at higher production rates can significantly increase the propensity for defects within the stretched filled film as well as creating other more serious manufacturing problems which can cause process line shutdowns. However, the ability to produce stretched filled films at increased rates is often desirable although any benefits to be achieved by increased rates of manufacture are reduced and/or entirely eliminated where the increased production rate also increase the level of defects and downtime of the production line.
Thus, there exists a need for an improved method of making stretched filled films. Moreover, there exists a need for a method of making stretched filled films which have reduced levels of defects and which are more tolerant of film irregularities during stretching. Further, there exists a need for such a method of making stretched filled films and, in particular a method of making breathable barriers, which allows for increased rates of production, improved efficiencies and wider processing windows.